Continuous gain compensation and fast band selection in a multi-standard, multi-frequency synthesizer

Abstract

A frequency synthesizer capable of high speed, low power, wideband operation including a method of gain compensation, and a method of fast voltage controlled oscillator (VCO) band calibration. In addition, the frequency synthesizer may include two or more switchable independent loop filters to facilitate wideband operation. Such a frequency synthesizer may be used in many applications, and in one example, may be particularly suitable for use in a multi-band, multi-standard transmitter or radio transceiver.

1 . A method of voltage controlled oscillator band selection in a frequency synthesizer, the method comprising acts of:
 setting a value of a band selection control signal to an initial setting based on an expected frequency band in which an operating center frequency is located;   iteratively adjusting the value of the band selection control signal to search one frequency band setting above and one frequency band setting below the initial setting until a proper setting for an operating frequency band in which the operating center frequency is located is determined; and   setting the value of the band selection control signal to the proper setting to tune a resonant frequency of the voltage controlled oscillator into the operating frequency band.   
   
   
       2 . The method as claimed in  claim 1 , further comprising an act of fine tuning the resonant frequency of the voltage controlled oscillator to the operating center frequency. 
   
   
       3 . The method as claimed in  claim 2 , wherein the act of setting the value of the band selection control signal includes setting a bit pattern for a digital control signal to control a plurality of switches to activate selected ones of a corresponding plurality of capacitors such that the resonant frequency of the voltage controlled oscillator is in the operating frequency band. 
   
   
       4 . The method as claimed in  claim 3 , wherein the act of fine tuning includes adjusting a control voltage for a variable capacitor to fine tune the resonant frequency of the voltage controlled oscillator to the operating center frequency. 
   
   
       5 . The method as claimed in  claim 1 , wherein the act of iteratively adjusting the value of the band selection control signal includes comparing a scaled version of the resonant frequency of the voltage controlled oscillator to a reference frequency using a race counter circuit. 
   
   
       6 . A voltage controlled oscillator comprising:
 a plurality of switchable tuning circuits that in combination provide a resonant circuit that generates a resonant frequency of the voltage controlled oscillator; and   a controller adapted to provide a digital band control signal that controls switching in and out of the resonant circuit the plurality of switchable tuning circuits to select an initial resonant frequency band setting;   wherein the controller is further adapted to iteratively adjust a value of the digital band control signal to search one frequency band setting above and one frequency band setting below the initial resonant frequency band setting until a proper value of the digital band control signal is determined to select an operating frequency band for the resonant circuit that includes a desired operating center frequency of the voltage controlled oscillator.   
   
   
       7 . The voltage controlled oscillator as claimed in  claim 6 , wherein the plurality of switchable tuning circuits comprises a plurality of switchable capacitors. 
   
   
       8 . The voltage controlled oscillator as claimed in  claim 7 , wherein the digital band control signal includes a plurality of bits and wherein the controller is adapted to set a bit pattern for the digital band control signal to control a plurality of switches to activate selected ones of the plurality of switchable capacitors such that the resonant frequency of the voltage controlled oscillator is in the operating frequency band. 
   
   
       9 . The voltage controlled oscillator as claimed in  claim 6 , further comprising a fine tuning circuit coupled to the plurality of switchable tuning circuits and to the controller, wherein:
 the controller is further adapted to provide a fine tuning signal to the fine tuning circuit to fine tune the resonant frequency of the voltage controlled oscillator to the desired operating center frequency.   
   
   
       10 . The voltage controlled oscillator as claimed in  claim 9 , wherein the fine tuning circuit includes at least one variable capacitor, and wherein the controller is adapted to adjust a control voltage for the at least one variable capacitor to fine tune the resonant frequency of the voltage controlled oscillator to the desired operating center frequency. 
   
   
       11 . A programmable two-point frequency synthesizer architecture comprising:
 a voltage controlled oscillator having a first port, a second port and an output;   a programmable divider coupled to the output of the voltage controlled oscillator and adapted to receive a data signal;   a phase detector having a first input coupled to an output of the programmable divider and a second input adapted to receive a reference frequency, the phase detector being adapted to produce a loop signal based on a combination of the reference frequency an a signal received from the programmable divider;   a first loop filter coupled between an output of the phase detector and the first port of the voltage controlled oscillator so as to provide a phase locked loop including the voltage controlled oscillator, the programmable divider, the phase detector and the first loop filter;   a variable gain amplifier having an output coupled to the second port of the voltage controlled oscillator, an input adapted to receive the data signal, and a control port; and   a correlation canceling circuit coupled to the control port of the variable gain amplifier and adapted to receive the data signal and the loop signal;   wherein the correlation canceling circuit is adapted produce a control signal based on the data signal and the loop signal and to apply the control signal to the control port of the variable gain amplifier; and   wherein the control signal is selected to continuously adjust a gain of the variable gain amplifier such that an output signal of the voltage controlled oscillator divided by the programmable divider is substantially equal to the reference frequency.   
   
   
       12 . The programmable two-point frequency synthesizer as claimed in  claim 11 , further comprising:
 a second loop filter coupled in parallel with the first loop filter between the output of the phase detector and the first port of the voltage controlled oscillator;   a first switch coupled to the first loop filter and adapted to switch in and out the first loop filter; and   a second switch coupled to the second loop filter and adapted to switch in an out the second loop filter;   wherein the programmable two-point frequency synthesizer is configured such that selective activation of the first and second switches causes one of the first and second loop filters to be active in the phase-locked loop.   
   
   
       13 . A frequency synthesizer comprising
 a voltage controlled oscillator coupled in phase-locked loop configuration with a programmable divider and a charge pump;   a first loop filter coupled between an output of the charge pump and an input of the voltage controlled oscillator;   a second loop filter coupled in parallel with the first loop filter between the output of the charge pump and the input of the voltage controlled oscillator;   a first switch coupled to the first loop filter and adapted to switch in and out the first loop filter; and   a second switch coupled to the second loop filter and adapted to switch in an out the second loop filter;   wherein the frequency synthesizer is configured such that selective activation of the first and second switches causes one of the first and second loop filters to be active in the phase-locked loop.   
   
   
       14 . The frequency synthesizer as claimed in  claim 13 , wherein the first and second switches are MOS switches. 
   
   
       15 . The frequency synthesizer as claimed in  claim 14 , wherein a value of a control voltage applied to a gate of the first switch is selected so as to open the first switch, thereby decoupling the first loop filter from the phase locked loop. 
   
   
       16 . The frequency synthesizer as claimed in  claim 13 , further comprising at least one additional loop filter coupled in parallel with the first and second loop filters; and a corresponding at least one additional switch coupled to the at least one additional loop filter and operable to connect and disconnect the at least one additional loop filter from the phase-locked loop. 
   
   
       17 . The frequency synthesizer as claimed in  claim 13 , wherein the first loop filter comprises a combination of resistors and capacitors selected and configured to implement a predetermined transfer function. 
   
   
       18 . The frequency synthesizer as claimed in  claim 13 , wherein the programmable divider is directly coupled to an output of the voltage controlled oscillator. 
   
   
       19 . The frequency synthesizer as claimed in  claim 18 , wherein the programmable divider comprises a plurality of cascaded fractional divider blocks; and wherein a digital control signal is applied to each of the plurality of cascaded fractional divider blocks to activate selected ones of the plurality of cascaded fractional divider blocks so as to set a divide ratio for the programmable divider. 
   
   
       20 . A method of controlling an operating frequency of a frequency synthesizer, the method comprising acts of:
 generating a resonant frequency using a phase-locked loop that includes a first loop filter and a second loop filter;   providing a selection signal that controls switching in an out of the phase-locked loop the first and second loop filters; and   adjusting the selection signal to control switching of the first and second loop filters, based on the resonant frequency, such that one of the first and second loop filters is active in the phase-locked loop.   
   
   
       21 . A programmable fractional-N divider comprising a plurality of fractional divider blocks coupled together in series, each one of the plurality of fractional divider blocks having a control port adapted to receive a digital control signal; wherein the digital control signal activates and deactivates selected ones of the plurality of fractional divider blocks so as to set a divide ratio for the programmable fractional-N divider; wherein:
 each of the plurality of divider blocks comprises a plurality of flip-flops coupled to digital components.   
   
   
       22 . The programmable fractional-N divider as claimed in clam  21 , wherein the digital components comprise at least one AND gate.